The typical prior art solenoid is one which controls the movement of the armature and obtains a force to move the armature by establishing an air gap which is changed. In most of the prior art solenoids the two pole pieces are flat and perpendicular to the path of movement of the armature. This is shown in U.S. Pat. No. 1,217,141 for example. In some prior art patents such as U.S. Pat. No. 750,132 the cooperating pole pieces are each tapered and this is primarily for the purpose of obtaining an air gap, for a given length of stroke, which is shortened proportional to the sine of the angle of the taper. This increases the force at the extended position of the armature. Rotary solenoids such as that shown in U.S. Pat. No. Re. 22,902 also utilize a tapered pole piece or a tapering amount of iron in the coal in order to achieve a pull on the armature tending to move the armature into the coil to a position whereat the greatest amount of iron is inside the coil.
Another form of the prior art is one wherein the armature may move beyond the axial center of the coil. This type is shown in U.S. Pat. No. 3,139,565 and the frame of the coil may have a magnetically permeable fixed pole piece extending axially part way into the bore of the coil. This axial extension is intended to carry all of the flux established by the coil.
Another prior art structure is suggested by U.S. Pat. No. 2,829,319 to have an armature with a hollow ellipsoid in the end so as to have, in effect, a tapering pole piece on the armature. This patent, however, specifically teaches that the flux established both by an electrical coil and a permanent magnet should be sufficiently low so that the pole piece is not saturated. Another prior art construction with a tapered pole tip is that shown in Control Engineering, November, 1974 at page 53. Such construction, however, effectively had both pole pieces on the stationary frame for a flux path therebetween and the cylindrical armature was movable to provide a second flux path from one pole piece to the other. The shaped pole piece was claimed to establish the armature position proportional to the input current rather than the usual function of a solenoid to be energized and to move quickly from an extended to a seated position. Still other prior art constructions were as shown in U.S. Pat. No. 2,357,959 wherein triangular magnetically permeable pieces were disposed in the air gap to be effectively in parallel with the flux between the pole pieces in the closed position of the armature. These were stated to provide an opposite or negative component to the force developed on the armature so as to reduce the acceleration of the armature and prolong the time delayed action of the solenoid.
The prior art solenoids have been ones wherein a DC operated solenoid is usually two to three times the volume of an AC operated solenoid for the same sealed force, so that in many cases an AC solenoid was used to save space even though these required shading coils or the like to avoid objectionable hum, and also required laminated silicon steel to avoid eddy current losses. The prior art AC solenoids often were made two to three times smaller than the DC solenoids for the same seated force, because they relied on the large inductive reactance of the AC coil, when the armature was seated, to limit the coil current. However, this usually meant that the AC solenoids were subject to having the electrical coil burn out if the armature of the solenoid were somehow prevented from seating within a very short time after the coil was energized. The typical prior art AC solenoid with a variable air gap was one which had a force versus stroke curve which was essentially an inverse square curve with the force increasing greatly just before the armature seated by engagement of the two pole pieces. This meant that a majority of the energy at this portion of the stroke is greatly in excess of the energy requirement of the attached working load. This large excess energy was absorbed by the mass of the load as kinetic energy and was dissipated in the destructive hammer blow shock occurring when the solenoid armature seated. Accordingly, the problem to be solved is how to construct a solenoid which avoids this large self-destructive hammer blow shock and makes the stroke versus force curve of the solenoid one which more closely approximates the stroke versus force requirements of the attached working load.